For well over a year, MI5 had been watching a group of young British Muslims after a tip-off from an informant.

Through an unprecedented surveillance operation involving bugging and phone tapping, they learned that in mundane residential streets a plot was being hatched which a senior security source described yesterday as "bigger than 9/11".

The plan, it appears, was to blow up passenger jets with liquid explosives hidden in hand luggage that would be combined on board with a detonating device, possibly hidden in an iPod.

In December 2005 the police joined the MI5 operation, with officers and agents monitoring what the alleged plotters read on the internet, where they spent their money, where they took their dry cleaning, which shops they used, and the meetings they attended as the authorities attempted to piece together what Peter Clarke, the head of Scotland Yard's anti-terrorist branch, described as the "aspirations of a large group".

As well intrusive surveillance, their spending habits and bank accounts were tracked by a special anti-terrorism unit which can monitor flows of money to provide evidence of association.

Sources say some in the alleged plot had access to unusually large amounts of money, out of keeping with their incomes.

But the group was not acting alone. It became clear last night that the trigger for the sudden police sweep on houses in Birmingham, High Wycombe and east London, and the arrest of 24 terror suspects, came from Pakistan.

The arrests in Britain followed the detention of terrorist suspects in Pakistan, it is believed, within the past fortnight. According to some government sources, after the arrests a message was sent to the suspected terror cells in Britain telling them: "Do your attacks now." In effect, it was a "go" order to the British bombers.

According to these sources, the message was intercepted and decoded by either British or US intelligence in the past 72 hours, spurring counterterrorism officials to intensify the investigation against the alleged plotters.

Mr Clarke suggested yesterday that the alleged bombers may not yet have created the bombs they intended to use.

"The devices were to be constructed in the UK," he said. But the plotters had given enough information in their overheard conversations to convince police the suicide attacks would be carried out using some form of liquid explosives hidden in bottles carried in their hand-luggage.

"The investigation has focused on intelligence, which suggested that a plot was in existence to blow up transatlantic passenger aircraft, in flight," he said.

"The intelligence suggested that this was to be achieved by means of concealed explosive devices smuggled on to the aircraft in hand baggage. The number, destination and timing of the flights that might be attacked remain the subject of investigation."

Security sources said up to 12 planes were to be targeted, suggesting at least two plotters were preparing to board each transatlantic jet.

Each person would have had a separate component of the explosive device, which on its own would have seemed harmless had the person been stopped and examined at security. This could avoid suspicion as the plotters passed through airport security, and suggests the planning had allowed for the terrorists being stopped and searched, but not being caught. Once the planes had taken off, the devices could then be assembled.

The sources say the power to detonate the devices could have come from iPods, laptops or mobile phones. The various components for the bombs were to be hidden in innocuous items such as soft drinks.

Though the alleged suicide bombers had been subject to intensive surveillance for several months, the full extent of what they were planning, what they were targeting and how they would attack only started to fall into place in recent days.

The US homeland security chief, Michael Chertoff, said yesterday that it was within the past fortnight that western intelligence agencies had realised the targets were US-bound planes leaving Britain.

In what may be a related move, Pakistani authorities yesterday placed Hafiz Mohammed Saeed, the founder of Laskhar-e-Tayyaba, the militant Islamist group that India accuses of sending hundreds of Pakistanis to fight in Kashmir, under house arrest.

Senior British anti-terrorist and security sources confirmed last night that they had been helped by the Pakistani intelligence agency, ISI.

However, they said that while the Pakistan link was an important one, it was just one factor in the decision to raid the houses of the suspects between 2am and 3am yesterday.

What is not in doubt is that the police moved before they had intended to. Senior anti-terrorism officials were called back from holiday to oversee the raids. Among them was the assistant commissioner of the Metropolitan police, Andy Hayman, who took an easyJet flight from Spain early yesterday morning.

"It was all very quick," a senior security source said.

At the same time, the Joint Terrorism Analysis Centre raised the terrorist threat level to the highest - critical - meaning "an attack is expected imminently", and indicating "an extremely high level of threat to the UK".

In addition to the 24 arrested yesterday police were still searching for three or four people involved in the plot, a senior counterterrorism official told the Guardian. They were not believed to be "main players" but officials could not rule out they had the capacity to inflict attacks within the UK.

As the 24 suspects were being prepared for questioning, the scale of what was being planned behind the facade of ordinary urban lives emerged.

US security officials believe their plot was a direct copy of the Bojinka operation in the mid-90s to bring down 12 western airlines simultaneously over the Pacific Ocean.

The US immediately accused al-Qaida of orchestrating the planned attacks. Mr Clarke was more circumspect. He said further investigations were needed to establish the exact destinations, and the timing of the flights that could have been attacked.

"During the investigation an unprecedented level of surveillance has been undertaken and our objective is to gather intelligence and evidence in support of the investigation," he said.

"As is so often the case in these investigations, the alleged plot has global dimensions."